Semiconductors, or computer chips, are found in virtually every electrical product manufactured today. Chips are used not only in very sophisticated industrial and commercial electronic equipment, but also in many household and consumer items such as televisions, clothes washers and dryers, radios, and telephones. As products become smaller but more functional, there is a need to include more chips in the smaller products to perform the functionality. The reduction in size of cellular telephones is one example of how more and more capabilities are incorporated into smaller and smaller electronic products.
A so-called “flip chip” is generally a monolithic semiconductor device, such as an integrated circuit, having bead-like terminals formed on one surface of the chip. The terminals serve to both secure the chip to a circuit board and electrically connect the flip chip's circuitry to a conductor pattern formed on the circuit board, which may be a ceramic substrate, printed wiring board, flexible circuit, or a silicon substrate. Due to the numerous functions typically performed by the microcircuitry of a flip chip, a relatively large number of terminals are required.
Because of the fine patterns of the terminals and conductor pattern, soldering a flip chip to a conductor pattern requires a significant degree of precision. Reflow solder techniques are widely utilized in the soldering of flip chips. Such techniques typically involve forming solder bumps on the surface of the flip chip using methods such as electrodeposition, by which a quantity of solder is accurately deposited on one surface of the flip chip. Heating the solder above a melting temperature serves to form the characteristic solder bumps. The chip is then soldered to the conductor pattern by registering the solder bumps with respective conductors, and reheating or reflowing the solder so as to metallurgically and electrically bond the chip to the conductor pattern.
Deposition and reflow of the solder must be precisely controlled not only to coincide with the spacing of the terminals and the size of the conductors, but also to control the height of the solder bumps after soldering. As is well known in the art, controlling the height of solder bumps after reflow is necessary in order to prevent the surface tension of the molten solder bumps from drawing the flip chip excessively close to the substrate during the reflow operation. Sufficient spacing between the chip and its substrate is necessary for enabling stress relief during thermal cycles, allowing penetration of cleaning solutions for removing undesirable residues, and enabling the penetration of mechanical bonding and encapsulation materials between the chip and the substrate.
A variety of methods are known in the art for controlling solder bump height. For example, the size of the exposed conductor area to which the solder bump is allowed to reflow can be limited. The approach involves the use of a solder stop, such as a solder mask or a printed dielectric mask, which covers or alters the conductor in the bump reflow region in order to limit the area over which the solder bump can reflow. A variation of the approach involves containing an I/O solder bump between and within a pair of recesses formed in the flip chip surface and the opposing surface of a substrate.
While several techniques for limiting and controlling height are accepted and used in the art, certain shortcomings remain.